Fix for cleaned URL.Path

Fix for #854

Strip [] from IPv6 addresses for FastCGI.

Fixes #845.

systemd, README: Edit to account for the recent spike in reports

We have had three operators within a few days which ran into the same cause
and had not been able to figure out what went wrong.

addresses #833, #822

fix typo

Provides some more guidelines to operators on how to avoid running Caddy as root.

Introduces an user www-data, which really is a placeholder. Such an user with the same UID/GID combination is created on the most popular Linux distribution. I trust any operator can spot the difference to his/her distro and adjust the unit file.

User nobody is not used here to avoid two easy pitfalls: Such an user should not be able to access private keys (for TLS), and should not write private keys (we would do that with Letsencrypt).

The exemplary unit file for systemd is intentionally redundant at times, for
example dropping privileges which an unprivileged user "www-data" did not have
in the first place: To aid as fallback in case the file gets copied and an
operator setting UID to 0 (which reportedly happened in the past).

browse: Decorate external links with: noopener noreferrer

Setting these on external links prefents the target from manipulating this
page by "window.opener" with some widely deployed browsers.

Restart gracefully with in-process restart

Adding note on the rationale for Go 1.6

Markdown changes