Rename `xuris.browser` key to avoid conflict

src/libexec/saturnin-xuris

 # choice of URI has been made; we may try to use dmenu
 bash -n <<<"$SATURNIN_XURIS_CHOICE"
 SATURNIN_XURIS_BIN=$(eval "$SATURNIN_XURIS_CHOICE")
-SATURNIN_XURIS_BIN="${SATURNIN_XURIS_BIN:-$(inigrep -j -1 -p xuris.browser)}"
+SATURNIN_XURIS_BIN="${SATURNIN_XURIS_BIN:-$(inigrep -j -1 -p xuris.default_browser)}"
 
 debug -v SATURNIN_XURIS_BIN