Assume that /proc/meminfo can be missing

Travis tests are failing, probably because /proc/meminfo is not available
in the test environment. The same might be true in some virtualized systems,
so just treat missing /proc/meminfo as a sign that hibernation is not
possible.
Remove duplicate entries from syscall list

ARM syscall list includes SYS_OABI_SYSCALL_BASE and SYS_SYSCALL_BASE
which were obsuring real syscall names.
transaction.c: do not point users to logs when unit not found

The logs are unlikely to contain any useful information in this case.

Also, change "walked on cycle path" to "found dependency on", which
is less technical and indicates the direction. With the old message,
I was never sure if prior units depended on later ones, or vice versa.

https://bugzilla.redhat.com/show_bug.cgi?id=996133
https://bugzilla.redhat.com/show_bug.cgi?id=997082
Verify validity of session name when received from outside

Only ASCII letters and digits are allowed.
build: check for build/link flags harder

Use AC_LINK_IFELSE instead of AC_COMPILE_IFELSE to test for flags that
might succeed during compilation but not during linking. An example is gcc
compiled with libssp support but gnu-ld without it. In this case
-fstack-protector works fine during compilation but fails during linking
as several internal helpers are missing.