server/client: add support for sending SIGUSR to footclient

This patch adds the IPC infrastructure necessary to propagate
SIGUSR1/SIGUSR2 from a footclient process to the server process.

By targeting a particular footclient instance, only that particular
instance changes theme. This is different from when targeting the
server process, where all instances change theme.

Closes #2156

doc/foot.1.scd

@@ -694,14 +694,14 @@ variables to unset may be defined in *foot.ini*(5).
 The following signals have special meaning in foot:
 
 - SIGUSR1: switch to color theme 1 (i.e. use the *[colors]* section).
-- SIGUSR2: switch to color theme 2 (i.e. use the *[colors2]* section)-
+- SIGUSR2: switch to color theme 2 (i.e. use the *[colors2]* section).
 
 Note: you can send SIGUSR1/SIGUSR2 to a *foot --server* process too,
 in which case all client instances will switch theme. Furthermore, all
 future client instances will also use the selected theme.
 
-Sending SIGUSR1/SIGUSR2 to a footclient instance is currently not
-supported.
+You can also send SIGUSR1/SIGUSR2 to a footclient instance, see
+*footclient*(1) for details.
 
 
 # BUGS

doc/footclient.1.scd

@@ -189,6 +189,21 @@ variables may be defined in *foot.ini*(5).
 In addition to the variables listed above, custom environment
 variables to unset may be defined in *foot.ini*(5).
 
+# Signals
+
+The following signals have special meaning in footclient:
+
+- SIGUSR1: switch to color theme 1 (i.e. use the *[colors]* section).
+- SIGUSR2: switch to color theme 2 (i.e. use the *[colors2]* section).
+
+When sending SIGUSR1/SIGUSR2 to a footclient instance, the theme is
+changed in that instance only. This is different from when you send
+SIGUSR1/SIGUSR2 to the server process, where all instances change the
+theme.
+
+Note: for obvious reasons, this is not supported when footclient is
+started with *--no-wait*.
+
 # SEE ALSO
 
 *foot*(1)