initial implementation of timerfd support

this builds but has not been tested. there are 3 todos listed
in the src/host/descriptor/shd-timer.c file that specify
some behaviors that should be tested on linux, since the manpage
does not specify what to do in some situations.

refs #217
added timerfd interposition and shadow glue code

refs #217
Merge branch 'master' of github.com:amiller/shadow into amiller-master
set flags for timerfd_create
Merge branch 'amiller-master'
Added pread

Signed-off-by: Andrew Miller <amiller@dappervision.com>