Don't truncate long commands in r_sys_cmdf and r_sys_cmd_strf ##util - #27303

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Description

Follow-up to #27287. r_sys_cmdf and r_sys_cmd_strf formatted into a char cmd[4096] and ignored what vsnprintf returned, so a longer command line was cut at 4095 bytes and whatever was left still ran. The effect depends on where the cut falls:

• Inside a quoted argument: sh rejects the whole command, so r_file_copy and r_file_move fail when the two escaped paths add up to more than 4 KiB.
• Between arguments: sh runs a shorter command, for example one without its trailing > file redirection.

Both now format with r_str_newvf and free the string after running it. They return -1 or NULL if formatting or the allocation fails, where master ran whatever was in the buffer. They do the same for a NULL fmt, as r_core_cmdf does.

On Windows, r_sys_cmd_strf can now pass command lines longer than 32 KiB to r_sys_create_child_proc_w32. #27302 fixes the buffer size there.

Testing

test_sys_cmdf_long in test/unit/test_util.c passes a 5000-byte argument through both: r_sys_cmdf with a redirection after it, and r_sys_cmd_strf capturing the output. I built with ./configure && make on Linux (gcc). On master the test fails at "r_sys_cmdf keeps the redirection after a long argument". With this change it passes, along with the rest of test_util. The other unit tests pass except the nine that need radare2-testbins or radare2 in PATH. sys/lint.py --all and git diff --check are clean.