Link up the interrupt frame to the systimer API. Use PGEX_U to indicate

Link up the interrupt frame to the systimer API.  Use PGEX_U to indicate
a user frame and adjust CLKF_USERMODE for the virtual kernel.  This allows
the stat clock in the virtual kernel to account for user vs system time
(instead of just counting it all as system time).

Consolidate other special virtual kernel and signal flags in tf_xflags and
name them the same (FPEX_FAULT -> PGEX_FPFAULT, and add PGEX_MAILBOX)

Implement PGEX_MAILBOX for signal frames.  On signal return this flag causes
P_MAILBOX to be set in order to maintain interlock semantics with mailbox
signals.