Bugfixes for checksum generation
This release fixes a potential double-free that may occur when a checksum file must be regenerated due to an added checksum type.
This double-free was observed at UCSD when CVMFS checksums were requested for files written prior to the CVMFS-style checksum support.
Further, this release fixes a potential deadlock in the SIGSEGV
signal handler due to prior use of async signal unsafe functions.