added cmake checks for pam_ext.h and pam_appl.h headers
added check for pam_syslog()
added pam_syslog() if doesn't exist
all cmake checks performed from inside the plugin
allow more characters in a valid user/group name:
* POSIX allows dashes '-' and dots '.'
* also the name may end with a dollar sign '$'
for our purposes it's enough to allow [-.$] anywhere in the name
