Borrowing from overlayfs's internal overlayfs.h, we define a
common "user.projection." extended attribute namespace prefix,
and then define our private empty flag attribute using it.

We expect to prepend our prefix to all user-defined projection
attributes, e.g., "user.projection.vfsforgit.contentid", etc.

Refactor our handling of the "empty" projection flag extended
attribute into two common base utility functions which get or set/remove
xattrs, and a specialized set of wrappers of these common functions
which get, set, or remove our projection flag attribute.

The common base functions catch the the ENOATTR "attribute not found"
error when reading or removing an attribute, and in this case, return
success but reset the caller's 'size' argument to -1 to signal that the
attribute was not found.

We will subsequently use this common pair of base functions to manage
additional, user-provided projection attributes, as will be required
for VFSForGit support.

We introduce a common iteration function for parsing and
applying user-provider lists of attributes, and use it to store
such attributes when they are provided during a call to
create a projected file or directory.

The iteration function will also be useful when we add the ability
for callers to subsequently read or reset these attributes after
the initial time of creation.

We store the name/value user attributes as xattrs under our
library's "user.projection.*" namespace, unless they would conflict
with our reserved "user.projection.empty" attribute; we allow
clients to potentially read that attribute, but not overwrite
or remove it.

When a normal setxattr() or removexattr() file operation is
invoked via our FUSE event loop, we check if it might alter
or remove one of the attributes we are managing within our
"user.projection.*" namespace, and if so, we deny the request.

While it is still possible for these xattrs to be modified
through changes made directly against our lower storage filesystem,
these checks should prevent inadvertent changes made through
our mount point.

Complete the implementation of our attribute API with
projfs_get_attrs() and projfs_set_attrs() function that permit
a client to read, set, reset, or remove any of their user-provided
projection attributes subsequent to the initial projection of
a file or directory.

Ensure the constructed user-provided xattr name is always
freed and doesn't leak memory, and separate name string
construction from its validation and use, per advice from @kivikakk.

Rename and reverse the sense of the xattr name check functions
to improve readability and code flow, per advice from @kivikakk.

Clarify the purpose of our projection-flag xattr handling
functions (which get/set/remove the "user.projection.empty"
extended attribute) by renaming to avoid the confusing "_empty"
suffix, per @kivikakk's suggestion.

Also use sizeof() to define the length of our reserved
xattr namespace prefix string, and align some whitespace.