Allowteamchange fix

Fixes "allowteamchange no" to actually work properly and NOT desync you from the game in most cases. You can now switch TO spectator freely (as intended it turns out), and switching FROM spectator is prohibited. The "Enter Game" menu option in Match/Tag now displays one of those blue window notices just like with "Change Teams" in CTF/Team Match if you cannot change teams right now. Everything else works as you'd expect.

The de-sync issue was originally reported here https://mb.srb2.org/showthread.php?p=789965#85

Can the menu option be disabled altogether when allowteamchange is off?

Not sure how simple that is to do, plus it'd look awkward if allowteamchange was disabled while you're looking at the menu possibly? (if you mean like it'd visibly vanish from the menu)

Added 15 new commits:

• 0f014fe2 - Merge branch 'master' into next
• b0f4bbb4 - Played TD's Stormy Streets enough to know precipitation sprites didn't get an ov…
• 5019077f - Merge branch 'precip-overflow-fix' into 'master'
• 2823c7bf - build: fixup warnings from GCC 6.2.1
• b22417bc - appveyor: buildbot now using GCC 6.3, not 5.3
• e0b2a4a7 - build: add suport for GCC 6.3
• b01d5da6 - build: fixup GCC54 endif
• 42ecca81 - build: disable tautological-compare and logical-op
• 81fe4621 - build: do not overwrite the -Wno-error switchs
• 6bb7a636 - appveyor: output commands passed to GCCwq
• 9cac1e9e - build: fixup WFALGS/WFLAGS mistake
• 4e8972cd - build: no warning or error about logical-ip or tautological-compare
• 538eac7a - appveyor: disable command output
• cb98c2d9 - Merge branch 'master' into next
• 7d25ab36 - Merge branch 'next' into allowteamchange-fix

Added 14 new commits:

• 8f4f8c6e - Merge branch 'master' into next
• b29193aa - CircleCI: first try
• 59d91e07 - build: r_bsp.c:213:23: warning: inlining failed in call to 'R_DoorClosed': call …
• c5d15ad5 - CircleCI: force -Wno-unsuffixed-float-constants
• ade354c2 - CircleCI: error on warnings
• aaaab40f - CircleCI: cache APT and ccache
• c85c277a - CircleCI: move GCC49 check to debian's env
• 99b2c888 - README: add CircleCI's Status badge
• ac75267e - CircleCI: build on Ubuntu as well
• 29c19b62 - CircleCi: Ubuntu docker image is broken
• 03ecb0d1 - CircleCI: add upx
• 52a79754 - CircleCI: keep build cache with checksum of depend.dep
• 2d68cb77 - Merge branch 'master' into next
• 4c992845 - Merge branch 'next' into allowteamchange-fix

Status changed to merged

mentioned in commit 0dabef73