signal.7: Describe EINTR semantics for recvmmsg(2)

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
signal.7: srcfix: Add FIXMEs

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
signal.7: Clarify text describing EINTR semantics for socket interfaces

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
man-pages.7: Note that .TH 'date' field is nowadays automatically updated by scripts

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>