builtin terminfo: add custom 'query-os-name'

Inspired by Kitty's 'kitty-query-os_name'. Notable changes:

* Drop kitty prefix
* os_name -> os-name
* Use "uname -s" without any transformations (e.g. no lower-casing)

    $ ./utils/xtgettcap query-os-name
    reply: (44 chars): <ESC>P1+r71756572792d6f732d6e616d65=4C696E7578<ESC>\
      query-os-name=Linux

Closes #2209

CHANGELOG.md

@@ -76,8 +76,11 @@
   Wayland protocol ([#2212][2212]).
 * `[colors-dark]` section to `foot.ini`. Replaces `[colors]`.
 * `[colors-light]` section to `foot.ini`. Replaces `[colors2]`.
+* `XTGETTCAP`: added `query-os-name`, returning the OS foot is
+  compiled for (e.g. _'Linux'_) ([#2209][2209]).
 
 [2212]: https://codeberg.org/dnkl/foot/issues/2212
+[2209]: https://codeberg.org/dnkl/foot/issues/2209
 
 
 ### Changed

README.md

@@ -641,6 +641,10 @@ All replies are in `tigetstr()` format. That is, given the same
 capability name, foot's reply is identical to what `tigetstr()` would
 have returned.
 
+In addition to queries for terminfo entries, the `query-os-name` query
+will be answered with a response of the form `uname=$(uname -s)`,
+where `$(uname -s)` is the name of the OS foot was compiled for.
+
 
 # Credits
 

scripts/generate-builtin-terminfo.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import argparse
+import os
 import re
 import sys
 
@@ -185,6 +186,7 @@ def main():
     entry.add_capability(StringCapability('TN', target_entry_name))
     entry.add_capability(StringCapability('name', target_entry_name))
     entry.add_capability(IntCapability('RGB', 8))  # 8 bits per channel
+    entry.add_capability(StringCapability('query-os-name', os.uname().sysname))
 
     terminfo_parts = []
     for cap in sorted(entry.caps.values()):