Move the Autoconf independent shell macros into the file

Move the Autoconf independent shell macros into the file
`shell.m4' and the name space `AS_*'.

* acgeneral.m4 (AC_SHELL_IFELSE, _AC_SHELL_TMPDIR, AC_SHELL_UNSET)
(AC_SHELL_MKDIR_P, AC_SHELL_DIRNAME): Rename and move to...
* shell.m4 (AS_IFELSE, AS_TMPDIR, AS_UNSET, AS_MKDIR_P)
(AS_DIRNAME): here, a new file.
Adjust all dependencies.

* tests/base.m4 (m4_wrap): Eve out into...
* tests/shell.m4: here, new file.
* tests/base.m4 (AC_SHELL_MKDIR_P)
(AC_SHELL_DIRNAME & AC_SHELL_DIRNAME_SED): Eve out into...
* tests/m4sugar.m4 (AS_MKDIR_P, AS_DIRNAME & AS_DIRNAME_SED):
here, new file.
Adjust the test suite.