gdb: add program_space parameters to some functions in symtab.c

Add some program_space parameters to functions related to getting and
setting the main name, making the references to current_program_space
bubble up a bit.  find_main_name calls ada_main_name, which implicitly
relies on the current program space, so I didn't add a parameter to that
function.

Change-Id: I9996955e8ae56832bbd461964d978e700e6feaf4
Approved-By: Tom Tromey <tom@tromey.com>