shells/pbosh: Update to 20200812

New features with AN-2020-07-18:
- Bourne Shell: A new symlink from /opt/schily/xpg4/bin/bosh to
  /opt/schily/xpg4/bin/sh is created when "smake install" is called.
  This helps people to call "bosh" in strict POSIX mode using the
  command line name "bosh" by putting /opt/schily/xpg4/bin in PATH
  before /opt/schily/bin, where the bosh implements better backwards
  cmpatibility to the Bourne Shell by default than a strictly POSIX
  compliant bosh would do. /opt/schily/xpg4/bin/bosh behaves the same
  as "/opt/schily/bin/bosh -o posix".

  Thanks to  Koichi Nakashima for reporting.

New features with AN-2020-08-12:
- autoconf: added a new test for the existence of <sys/auxv.h>

- autoconf: added new tests for Linux getauxval() and FreeBSD elf_aux_info()

- libschily: getexecpath.c now uses getauxval() on Linux and elf_aux_info()
  on FreeBSD. This was needed since readlink("/proc/curproc/file") on
  FreeBSD returns random values for hardlinked files, making it impossible
  to use the result in order to find out which behavior variant of a
  fat binary is requested.

- Bourne Shell: Several #ifdef SIG* have been added to make it compile
  on older UNIX versions.

  Thanks to a hint from Albert Wik.

(micha)