improved the regexp for disabling password login

[mastak]

By default PasswordAuthentication is:

# PasswordAuthentication yes

Regexp like ^(#\s*)?PasswordAuthentication handles all possible variants and skip mention in comments:

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.

[vitalk]

Thank you, @mastak!