This patch changes our use of fgets() to tor_fgets() for more consistent
error handling across different versions of the C library.

This patch adds the `tor_fgets()` function to our compatibility layer.

`tor_fgets()` adds an additional check for whether the error-bit have
been enabled for the given file stream, if that is the case and `errno`
is set to `EAGAIN` we make sure that we always return NULL.

Unfortunately `fgets(3)` behaves differently on different versions of
the C library.

See: https://bugs.torproject.org/21416
See: https://bugs.torproject.org/20988

George Kadianakis authored 8 years ago and Nick Mathewson committed 8 years ago

- Make sure we check at least two guards for descriptor before making
  circuits. We typically use the first primary guard for circuits, but
  it can also happen that we use the second primary guard (e.g. if we
  pick our first primary guard as an exit), so we should make sure we
  have descriptors for both of them.

- Remove BUG() from the guard_has_descriptor() check since we now know
  that this can happen in rare but legitimate situations as well, and we
  should just move to the next guard in that case.

"Ours" merge to avoid version bump

Apparently some unixes don't like this.
Part of #21581.

Part of 21581.

(But use bash if it's available.)
This is a workaround until we remove bash-specific code in 19699.

Fixes bug 21581; bugfix on 21562, not in any released version of tor.

This is an "ours" merge to avoid a version bump.

This is an "ours" merge to avoid a version bump.

This is an "ours" merge to avoid taking a version bump

This is an "ours" merge to avoid taking a version bump, and to
avoid replaying the post-0.2.7.6 history of "maint-0.2.7-redux" onto maint-0.2.8, which already included the relevant changes.

"ours" merge to avoid version bumps

"ours" merge to avoid version bumps

"ours" merge to avoid bumping version