Fixed uninitialized 'pw' variable in HAS_SHADOW logic in session.c due

to flaw in initial integration.  Enabled HAS_SHADOW for Solaris, which
always has shadow password files.  Fixed all of pppd to compile with gcc
-Wall on Solaris.  (Still need fixes for kernel modules; blocking
Makedefs.gcc update.)  Tested with and without PAM, with gcc and Sun's
cc.