These have no use, exception for creating an artificial dependency
from input_plugins[] to all archive plugins.

These methods are not exception-safe; std::lock_guard should better be
used.

Don't keep any references to the owner, as the handler method may
delete the owner.

Fixes search within mount points, resulting in error "No such
directory".

Closes #190

libicu.a depends on libutil.a.

No longer a problem with NDK r16.

Android native code should be position-independent.

The NDK build scripts use "-fpic" instead of "-fPIC" for ARM, but that
doesn't work with FFmpeg's assembly code, because it requires
R_ARM_MOVW_ABS_NC which is unavailable with "-fpic".

By telling clang which implementation to use, we avoid the dependency
on libstdc++.so.

The caller should be able to safely store the instance pointer before
his callback is invoked.

The duration of a song can have fractions of seconds
(quote from http://www.upnp.org/schemas/av/didl-lite-v2.xsd):

    The format of the duration string is:
    H+:MM:SS[.F+], or H+:MM:SS[.F0/F1]
    Where:
    +H              one or more digits to indicate elapsed hours,
    MM              exactly 2 digits to indicate minutes (00 to 59),
    SS              exactly 2 digits to indicate seconds (00 to 59),
    F+              any number of digits (including no digits) to indicate fractions of seconds,
    F0/F1   a fraction, with F0 and F1 at least one digit long,
                    and F0 < F1.
    The string may be preceded by an optional + or - sign, and the
    decimal point itself may be omitted if there are no fractional  seconds digits.

Until now, a duration with fractions of seconds could not be parsed and
resulted in an unknown duration. Only durations in the format "H+:MM:SS"
were feasible. This commit enables to read durations in the first format,
i.e. "H+:MM:SS[.F+]"