gdb: improve show text and help text for 'remote exec-file'

gdb: improve show text and help text for 'remote exec-file'

The current behaviour for 'show remote exec-file' is this:

  (gdb) show remote exec-file

  (gdb) set remote exec-file /abc
  (gdb) show remote exec-file
  /abc
  (gdb)

The first output, the blank line, is just GDB showing the default
empty value.

This output is not really inline with GDB's more full sentence style
output, so in this commit I've updated things, the output is now:

  (gdb) show remote exec-file
  The remote exec-file is unset, the default remote executable will be used.
  (gdb) set remote exec-file /abc
  (gdb) show remote exec-file
  The remote exec-file is "/abc".
  (gdb)

Which I think is more helpful to the user.

I have also updated the help text for this setting.  Previously we had
a set/show header line, but no body text, now we have:

  (gdb) help show remote exec-file
  Show the remote file name for starting inferiors.
  This is the file name, on the remote target, used when starting an
  inferior, for example with the \"run\", \"start\", or \"starti\"
  commands.  This setting is only useful when debugging a remote target,
  otherwise, this setting is not used.
  (gdb)

Which I think is more helpful.

Reviewed-By: Mark Wielaard <mark@klomp.org>
Tested-By: Mark Wielaard <mark@klomp.org>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>