check_icmp: Support "--help" and "--version"

Let check_icmp provide "--help" and "--version" flags (in addition to
"-h" and "-V"), just as the other plugins do.
check_icmp: Minor whitespace/indentation fixes
Update bundled libtool files

Import the current libtool version 2.4.2.
check_snmp: Initialize size_t value to 0, not NULL
s/--proxy_authorization/--proxy-authorization/

The check_http option "--proxy_authorization" is now called
"--proxy-authorization" instead.  It was always documented this way, and
we don't usually use underscores in option names.
check_tcp: Don't close connection too early

Closing the connection because the bytes received are less than the
buffer size assumes that all the bytes will be received in one go.  This
is not always true!