Extend cmd line processor to deal with envar ops

PMIx supports forward/set, unset, append, and prepend of
environmental variables. However, the cmd line processor
wasn't setup to handle cmd line options for that purpose.
Extend it to provide such support.

Forward (-x) of envars can be just the envar name (to pickup
the local value and forward it), or can be envar=value to
set the envar to a specific value.

Unset (--unset) takes just the name of the envar.

Append (--append-env) takes two arguments:

• the name of the envar, appended with a "[c]" where
  the 'c' is the character to be used as the separator
  between envar values
• the value to be appended
  So it looks like "--append-env FOO[:] 20"

Prepend (--prepend-env) behaves exactly like append except
it prepends the value to whatever current envar value it finds

Multiple instances of any of these options may be present on
the cmd line. Each instance will have its arguments appended
to the parameter's pmix_cli_item_t's values argv-array.

Signed-off-by: Ralph Castain rhc@pmix.org
(cherry picked from commit c4694cb)

Roll version to v5.0.8

Signed-off-by: Ralph Castain rhc@pmix.org
bot:notacherrypick