Remove BSTRING related code, all platforms define it.

Remove BSTRING related code, all platforms define it.
* src/s/usg5-4.h (BSTRING): Remove definition.
* src/s/template.h (BSTRING):
* src/s/msdos.h (BSTRING):
* src/s/ms-w32.h (BSTRING):
* src/s/hpux10-20.h (BSTRING):
* src/s/gnu-linux.h (BSTRING):
* src/s/darwin.h (BSTRING):
* src/s/cygwin.h (BSTRING):
* src/s/bsd-common.h (BSTRING):
* src/s/aix4-2.h (BSTRING): Likewise.
* src/sysdep.c: Remove code depending on BSTRING not being defined.

* configure.in: Remove code dealing with BSTRING.

* lib-src/ntlib.h: Remove code dealing with BSTRING.

* nt/config.nt: Remove code depending on BSTRING.