Move the symbol-file-from-memory functions into their own file.

Move the symbol-file-from-memory functions into their own file.
* symfile-mem.c, symfile-mem.h: New files.
* symfile.c (symbol_file_add_from_bfd): New function.
(symbol_file_add): Call symbol_file_add_from_bfd.
(symbol_file_add_from_memory, add_symbol_file_from_memory_command):
Moved to symfile-mem.c.
(_initialize_symfile): Move definition of
add-symbol-file-from-memory command to symfile-mem.c.
* symfile.h (symbol_file_add_from_bfd): New declaration.
* config/i386/linux.mt (TDEPFILES): Add symfile-mem.o.
* config/powerpc/linux.mt (TDEPFILES): Same.
* Makefile.in (SFILES): Add symfile-mem.c.
(symfile_mem_h): New variable.
(HFILES_NO_SRCDIR): Add symfile-mem.h.
(symfile-mem.o): New rule.