Simplify start_of_data, start_of_text and related code.

Simplify start_of_data, start_of_text and related code.
* src/mem-limits.h: Remove !emacs and _LIBC conditional code.
(start_of_data): Merge into start_of_data function.
* src/sysdep.c (start_of_text): Remove.  Move simplified versions of
it in the only users: src/unexaix.c and unexec.c.
(read_input_waiting): Remove local declaration of quit_char.
(start, etext): Remove declarations.
(start_of_data): Merge with the version in mem-limits.h and move
to vm-limits.c.
* src/vm-limit.c (start_of_data): Merged and simplified version of the
code formerly in mem-limits.h and sysdep.c.
* src/unexec.c (start): New declaration, moved from sysdep.c.
(start_of_text): Simplified version of the code formerly in sysdep.c.
* unexaix.c (start_of_text): Simplified version of the code
formerly in sysdep.c.
* src/m/alpha.h (HAVE_TEXT_START): Remove.
(TEXT_START): Move ...
* src/unexalpha.c (TEXT_START): ... here.
* src/s/hpux10-20.h (TEXT_START): Remove.
* src/s/darwin.h (TEXT_START):
* src/m/mips.h (TEXT_START):
* src/m/macppc.h (HAVE_TEXT_START):
* src/m/m68k.h (TEXT_START):
* src/m/iris4d.h (TEXT_START):
* src/m/intel386.h (TEXT_START):
* src/m/ibmrs6000.h (TEXT_START):
* src/m/ia64.h (HAVE_TEXT_START):
* src/s/msdos.h (TEXT_START): Likewise.