Bump Version to 1.5.1 (#138)

Changes since 1.5.0:

• improve/fix cmdline handling
  • tell the actual watchdog device specified with -w
  • tolerate and strip any leading spaces of commandline option values
  • Sanitize numeric arguments
• if start-delay enabled, not explicitly given and msgwait can't be read from disk (diskless) use 2 * watchdog-timeout
• avoid using deprecated valloc for disk-io-buffers
• avoid frequent alloc/free of aligned buffers to prevent fragmentation
• fix memory-leak in one-time-allocations of sector-buffers
• fix AIO-API usage: properly destroy io-context
• improve/fix build environment
  • validate configure options for paths
  • remove unneeded complexity of configure.ac hierarchy
  • correctly derive package version from git (regression since 1.5.0)
  • make runstatedir configurable and derive from distribution