* nt/configure.bat: Renamed the fusercflags variable to escusercflags

* nt/configure.bat: Renamed the fusercflags variable to escusercflags
so that the variable name better matches its purpose, to be identical
to usercflags with the exception that all quotes are escaped by the \
character.

Renamed the fuserldflags variable to escuserldflags so that the
variable name better matches its purpose, to be identical to
userldflags with the exception that all quotes are escaped by the \
character.

A new ESC_USER_CFLAGS variable is written to config.settings.  This
variable has the same value as the escusercflags variable.

* nt/gmake.defs, nt/nmake.defs: Added the variable ESC_CFLAGS.  This
variable is identical to the CFLAGS variable except that it includes
the new ESC_USER_CFLAGS variable instead of USER_CFLAGS.

* src/makefile.w32-in: The bootstrap-temacs rule now makes use of one
of two shell specific rules, either bootstrap-temacs-CMD or
bootstrap-temacs-SH.  The bootstrap-temacs-SH rule is identical to the
previous implementation of the bootstrap-temacs rule.  The
bootstrap-temacs-CMD rule is similar to the previous implementation of
the bootstrap-temacs rule except that it makes use of the ESC_CFLAGS
variable instead of the CFLAGS variable.

These changes are required to extend my earlier fix to add support for
--cflags and --ldflags options that include quotes so that it works
whether make uses cmd or sh as the shell.