debian: fix compatibility with systemd 215 on Jessie

Since FS-8788, this error is printed to the journal:

    [/lib/systemd/system/freeswitch.service:22] Failed to parse resource value, ignoring: 240K

It turns out that suffixes are only introduced in systemd 228 which is
not available in Debian Jessie. Use 240*1024, rounded up to a nice
human-readable number.

debian/freeswitch-systemd.freeswitch.service

@@ -19,7 +19,7 @@ Group=daemon
 LimitCORE=infinity
 LimitNOFILE=100000
 LimitNPROC=60000
-LimitSTACK=240K
+LimitSTACK=250000
 LimitRTPRIO=infinity
 LimitRTTIME=7000000
 IOSchedulingClass=realtime