SERVER-9786 Link to strcasestr at runtime for Solaris

For the Solaris/SmartOS build, do not make direct calls to strcasestr,
which is present in Solaris 11 but not in Solaris 10.  Instead, see if
it is available in a loaded library (which will be libc.so.1) at runtime
and either call it or call an emulation.  Use the emulated version on
Windows, which is also missing this function.
SERVER-9832 Use PF_UNIX symbol instead of PF_LOCAL

PF_LOCAL is the POSIX name for the Unix domain sockets protocol family,
while PF_UNIX is the name that was used by BSD.  Solaris 10 declares
PF_UNIX but not PF_LOCAL, so use that symbol instead.