Add support to parse dmesg and workqueue output with 3.18 kernel

Change-Id: If31d4e7512f9da8a9f14fc6814fe57aab05d978d

Requires CONFIG_LOG_BUF_MAGIC; otherwise no error checking will be
performed.

A log record is valid if:
1) The magic value matches
2) the length recorded in the header matches the sum of the lengths of the
subelements.

If a log record fails this check, perform a linear search
to find the next matching log record.

Change-Id: I584350c8bc1bf93c2d956fa1e05db8207cc2d40d

Kernel change 62e32ac3505a0cab1c5ef8ea2c0eab3b26ed855f
renamed 'struct log' to 'struct printk_log'. Detect which kernel
version is being parsed and use the appropriate struct name.

Change-Id: I1a92623e8b4167f390d09051a52e6411ae3901ee

It can be useful to extract the dmesg from contexts other than the
--dmesg parser. Refactor the code into a top-level "library" module and
hook the --dmesg parser into that.

Change-Id: I935c05a081b1096d67343f5ce8617febb674b2e0