ppp: treat peer's lcp echo request as lcp echo activity and don't drop connection if there is no peer's echo reply

use /proc/sys/fs/nr_open in conjuction with /proc/sys/fs/file-max to determine limit of maximum number of opened files

log_file: write authentication failed session logs to different file (optional)

ppp: default fsm timeout increased to 3 sec.
ppp: accept LCP TermReq/TermAck packets in terminating phase

shaper_tbf: fixed bug of incorrect operation when time ranges are spicified and connection whithout time range identifier is arrived