v1.7.17

New transfer modification features for sync and direct I/O, a minor display formatting fix, extensions to the test framework, and the first round of cleanups.

• feature: new "--sync" option to flush cache to disk after every write (related to GH#6, to improve accuracy when writing to slow disks)
• feature: new "--direct-io" option to bypass cache - implements GH#29 "Option to enable O_DIRECT" - requested by Romain Kang, Jacek Wielemborek
• fix: correct byte prefix size to 2 spaces in rate display, so progress display size remains constant at low transfer rates
• cleanup: rewrote configure.in as per suggestions in newer "autoconf" manuals
• cleanup: replaced header.in with one generated by "autoheader", moving custom logic to a separate header file "config-aux.h"
• cleanup: added copyright notice to all source files as per GNU standards
• cleanup: changed "--version" output to conform to GNU standards
• cleanup: replaced backticks with $() in all shell scripts that did not come from elsewhere, as backticks are deprecated and harder to read
• cleanup: improved the output formatting of "make test"
• cleanup: extended the "make test" mechanism to allow certain tests to be skipped on platforms that cannot support them
• cleanup: skip the "pipe" test (for SIGPIPE) if GNU "head" is not available, so that "make test" on stock OpenBSD 7.3 works
• cleanup: added a lot more tests to "make test"
• cleanup: replace all calls to sprintf() and snprintf() with a new wrapper function pv_snprintf() to improve security and compatibility
• cleanup: replace all calls to strcat() with a wrapper pv_strlcat() to improve security and compatibility
• cleanup: replace all write() calls to the terminal with a wrapper pv_write_retry() for consistency
• cleanup: tidy up and fix compilation warning in "--watchfd" code
• cleanup: rewrote all local shell scripts to pass analysis by ShellCheck