gdb/doc: add missing parentheses around prompt in some examples

While reading the manual for -info-os I noticed that the GDB prompt is
given as 'gdb' when it should really be '(gdb)'.  This is because the
prompt is created with: '@value{GDBP}'.

The GDBP variable (the GDB program name) is intended for use as the
prompt string (though this is not used consistently throughout the
manual), however it is normally used like '(@value{GDBP})', but in a
couple of places the enclosing parentheses are missing.

In this commit I do the following:

 - Change '@value{GDBP}' to '(@value{GDBP})' wherever the variable
   represents a prompt string.

 - Replaces '(gdb)' with '(@value{GDBP})' in one example where we are
   already using '(@value{GDBP})', this makes that one example
   consistent.

I have NOT:

 - Changed all instances of '(gdb)' with '(@value{GDBP})', this would
   be a huge change.

gdb/doc/ChangeLog:

	* gdb.texinfo (GDB/MI Miscellaneous Commands): Add missing
	parentheses to GDB prompt in example, and replace '(gdb)' with
	'(@value{GDBP})' in one example where the latter was already in
	use.