[stream-file] Keep head value in memory

Reduces syscalls.

Part of Bug 637275 - Use pread/pwrite() in vtestream-file.h if available
[stream-file] Bug 637275 - Use pread/pwrite() if available

Turns syscalls in half.
[stream-file] Mark temp files NOATIME
[ring] Rename variables for readability

Patch from Egmont Koblinger.

Part of Bug 708496 - Lines disappear after positioning back the cursor
Bug 708496 - Lines disappear after positioning back the cursor

Patch from Egmont Koblinger.
Fixup commit a551b0b8686bc266a2329a3215069895d591b0e9
[stream-file] Remove another lseek
[stream-file] Recover from a disk-full situation
stream: file: Use O_TMPFILE when available

https://bugzilla.gnome.org/show_bug.cgi?id=704705
(cherry picked from commit c80fe16c3019eeb0686023b2968199e4d84a2676)
prompt: Set LC_ALL instead of LANG

LC_ALL and LC_* override LANG, so if the user has any of them set,
__vte_urlencode would give the wrong output.

https://bugzilla.gnome.org/show_bug.cgi?id=709354
(cherry picked from commit 9c9f1451f37864f18e711c348f6335a22e7c4c4d)
portability: protect O_NOATIME

The O_NOATIME open() flag is only available on Linux, so unbreak building
by protecting it with an ifdef.