Add + expose pselect

EDIT: Don't use 0.1.18 - recommend 0.1.17 until I've fixed the problems.

Previous notes:

Additional posix function pselect (needed by p11-kit).

0.1.17 - expose getopt_long impl

getopt_long was already in the library but not exposed in headers. Hopefully this can then be used where packages don't include a replacement already (libtasn1 is the motivator).

Add strsignal implementation

Add strsignal implementation + header prototype.

updates for strstr and strcasestr

0.1.15

Typo + better example for use in readme.

Include mempcpy prototype

0.1.14

Add missing mempcpy prototype to string.h

Make dicl headers compatible with C++ code.

0.1.13

Fix up dicl headers to be able to import into C++ code too.

Fix sgi mips endianness definition.

0.1.12

Fix the endianness. sgi is big endian.

Add faccessat last needed rpm compat.

0.1.11

Missed update to configure.ac

Lots of new posixy confirmant goodness in 0.1.10

A bunch more stuff from gnulib for 0.1.10

Bug fix, ensure allocate space for errno msg.

0.1.9

Bug fix - ensure allocation of %m format