"Not charging" is 12 characters; %12[a-zA-Z ] reads up to 12 chars
plus a NUL terminator (13 bytes) into a 12-byte buffer. Increase
state buffer to 13 in both battery_state and battery_remaining.

ifindex() caches the result of ioctl(SIOCGIFINDEX) and never
retries it, since the interface name strcmp succeeds after the
first call. if the interface does not exist when slstatus starts
(e.g. iwd with default config destroys/recreates interfaces
depending on service state), the ioctl fails and the stale
result is returned on all subsequent calls.

based on a patch by ryhpez, with input from Taha Medhous

more flexible parsing for /proc/meminfo to take shared and reclaimable
memory into account. this matches the output with free(1).

additionally this could fix some corner cases, as the order of fields in
/proc/meminfo is not strictly defined.

slstatus:
percent 81% free 2.5 Gi total 23.4 Gi used 19.0 Gi

free(1):
               total        used        free      shared  buff/cache   available
Mem:            23Gi        19Gi       2.5Gi       1.3Gi       3.2Gi       3.6Gi

Signed-off-by: drkhsh <me@drkhsh.at>

opendir() returns a directory stream, not a file descriptor

Co-authored-by: drkhsh <me@drkhsh.at>
Signed-off-by: drkhsh <me@drkhsh.at>