Check template validity before trying to build droids.
Polish translation update by Emdek.

Closes #1347.
French translation update by Amiral JCDG.

Closes #3483.
Hungarian translation update by mtomi78.

Closes #3485.
Turkish translation update by Ayhan Görgülü.

Closes #3486.
Dutch translation update by nino_romans.

Closes #3484.
Fix some typos.

Thanks to pabs for noticing one of them. Closes #3495.
fix an FTBFS with -Werror=format-security

Fixes ticket:3493.
Fix flag loading code to work for out of sequence factories.

fixes ticket:3384
Merge branch 'bugfixes'
Fix communication between remaining clients when one client freezes.

Problem persisted until the frozen client drops completely, or unfreezes (net connection is restored).

To reproduce, test with host and 2 other clients, and freeze one client with kill -STOP or by pausing the net
connection somehow.

Cause was lack of flushing, due to an inverted condition exposed in b9cc7702b7dc7ff3781f32f69b9cac110ebd6b98 and
introduced in c1ca4d2d343e01ab9eb672bf6d3b2809e11efe19. Would probably trigger before
b9cc7702b7dc7ff3781f32f69b9cac110ebd6b98, for any remaining clients that freeze for any reason (for example, because
of <2.5 FPS or momentary hard drive swapping) for more than 400ms.
Be more consistent in ping display, show ping ≥ 4000 as ∞.
Put the unused MAX_BYTESPERSEC define out of its misery.
Fix Nexus AI unable to build units.

Nexus was building units with an unresearched sensor (equivalent to the default sensor). Since
6365f2dbec51abb8ac2509c0623b54dce3e655fd, Nexus can no longer build unresearched units.

In the tutorial, the designed MG viper wheels would also use the same unresearched sensor.

Replaced unresearched DefaultSensor1Mk1 with ZNULLSENSOR. Removed special case for tutorial in
7079ad295112abcd83e6be1e7c172d36aec64c25, which is no longer required.
Fix unconventional reported unit count on selecting all units of same type.

Previous unconventional behaviour was to consider the set of previously-unselected droids becoming selected, and
report the size of the set plus the size of the set of types of droids in the set.

Iff there was exactly one droid of each type previously selected, and there was at least one unselected droid of
each selected type, then the reported unit count would coincidentally be correct.

Fixes ticket:3479.
Show "[PlayerName] has left the game." to all players again.

Fixes ticket:3461.
Fix build, missing function parameter after merge.

Broken in dadfdf00f40292af947b5ee7cba9408162e6200a, due to interaction between
4f030f4b6de8eb6ca9fb422f4b9548ca6127acb8 and 6365f2dbec51abb8ac2509c0623b54dce3e655fd.
Update zlib to 1.2.7.
Don't spam assertions while artillery is assigned to allied sensors.
Let CB sensors defend allies too.