Introduce exec_file_locate_attach

Introduce exec_file_locate_attach

This commit adds a new function, exec_file_locate_attach, which works
like exec_file_attach except that, instead of a filename argument, it
takes an integer process ID and attempts to determine the executable
filename from that.

gdb/ChangeLog:

	* gdbcore.h (exec_file_locate_attach): New declaration.
	* exec.c (exec_file_locate_attach): New function, factored
	out from...
	* infcmd.c (attach_command_post_wait): ...here.