hostapd: re-introduce process tracking

Before commit 60fb4c92 ("hostapd: add ubus reload") netifd was
tracking hostapd/wpa_supplicant and restarting wifi in case of a
process crash. Restore this behaviour by tracking the PIDs of
hostapd and wpa_supplicant.
Also make sure hostapd and/or wpa_supplicant have been started before
emmitting ubus calls to them using ubus wait_for.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>